The present invention relates to a phase mask for forming diffraction grating, and to an optical fiber and an optical waveguide having Bragg diffraction grating fabricated using the phase mask. The invention especially relates to a fabrication method of a phase mask for forming diffraction grating in optical waveguide and optical fiber used in the optical communication and other uses, by means of ultraviolet light, and to an optical waveguide and an optical fiber having Bragg diffraction grating fabricated using the phase mask.
In optical fiber communication system, it is necessary to transmit a large volume of information at very high speed. An optical fiber system comprises an optical signal source for carrying information, an optical fiber transmission line, and a receiver for demodulating information carried by optical signals.
In optical fiber communication system, a silica optical fiber with high purity is generally used as transmission medium. Conventionally, optical communication systems have been designed to use a certain wavelength range for transmitting optical signals. In such systems, transmission delay occurs depending on wavelength. In the past, however, this dispersion did not significantly deteriorate the information of optical signals. This is because in the early systems, a single channel was used which was selected from a wavelength range having small dispersion.
In these days, however, it is required to use a number of channels ranging in a wide wavelength region in order to carry more information, as is the case in wavelength division multiplexing (WDM) system, which is one of the high speed transmission techniques using optical fiber. This in turn requires a more precise compensation of the group velocity dispersion. For instance, in relation to WDM system, the dispersion compensation has become an increasingly important problem as the number of channels provided in WDM system increases.
Further, as the Bragg diffraction grating formed within optical fiber, a chirped grating has come to be required recently, in which the grating pitch increases or decreases linearly or nonlinearly depending on the position of the grating in a direction perpendicular to the grating pitch (the direction along which the grating is repeated) These gratings are used in such uses as high reflection mirror with enlarged reflection band and delay adjusting means.
Corresponding to above described increasing requirement to the chirped grating, the inventor of the present invention has proposed a method for fabricating phase mask for processing optical fiber. According to this method, a transparent substrate comprises on its one surface a repetitive pattern of grooves and strips located in a grating form, so that an optical fiber is irradiated with diffracted light according to the repetitive pattern so as to make a diffraction grating within the optical fiber by an interference fringe of diffracted lights of different orders, wherein a plurality of patterns comprising differently pitched grooves and strips are imaged in a juxtaposed relation, thereby fabricating the repetitive pattern of grooves and strips located in a grating form (Japanese Unexamined Patent Publication No. H11-72631).
In conventional fabrication method of the mask for forming nonlinear chirped grating used for the purpose of dispersion compensation, the rate of change of the cycle on the mask varies depending on section. In this case, in such region with a large variation in the rate of change of the cycle depending on section, deterioration of performance such as increase of ripple (fluctuation) in the group delay characteristics may be caused.